1. Field of the Invention
The present innovation relates to the field of metallographic process control engineering. More specifically, it constitutes an apparatus and technique for the in situ monitoring and control of grain size as well as other microstructural variables, e.g., grain orientation, in the production of steel alloys.
2. Description of Prior Art
It has long been known to those skilled in the art that grain size is an important predictor of desirable metallurgical properties in steel products. Ductility, yield strength, tensile strength, and other physical properties of steel alloys can be predicted and controlled by the manipulation of polycrystalline grain size. In nonoriented electrical steels used in motors and transformers, etc., controlling grain size is extremely important in the production of energy efficient alloys with low "iron loss". In oriented electrical steels both grain size and orientation are important in producing a low loss alloy.
Chemical, thermal, and mechanical conditions during the milling process can greatly influence the final grain texture of the material. Specifically, mechanical deformation interacting with complex thermal sequences in particular chemical and gas atmospheric environments determines the ultimate microstructural characteristics of the metal alloy.
At the present time there is no sensor that can monitor microstructural transitions in situ for rapid metal forming processes like the production of steel. The highest volume product in the steel industry is hot rolled strip (30% of all steel shipped) which moves off the line at speeds in excess of 4500 feet per minute. The first opportunity to observe microstructural properties like grain size occurs after the fact when samples are removed from the coiled steel and sent to the laboratory for inspection by optical microscopy. If processing conditions were not optimal at any point in the milling process, an entire heat may have to be diverted to alternative use, given remedial heating, or melted down for scrap at substantial cost to the producer.
Determination of grain texture using optical microscopy to inspect polished and etched samples is a tedious and time consuming process. Thus the testing of hot and cold rolled strip, for instance, is typically limited to the inspection of samples cut from the head and tail of each coil. If on-line grain size measurements were possible, it would not only make it easier to accurately control production, but it would also permit exploration of the heuristic consequences resulting from fine tuning the complex mechanical, chemical, and thermal interactions inherent, but at this point invisible, in the steel manufacturing process.
Techniques have been described using ultrasound backscattering and X-ray diffraction to measure grain size on-line. In the case of ultrasound the time constants involved in the propagation and analysis of sound waves traveling through the steel are too long to determine grain size in the rapid metal forming processes commonly used in the production of hot or cold rolled strip. In the case of X-ray diffraction the temporal requirements of proportional X-ray counters currently available make any time-integrated intensity method, while theoretically possible, very impractical for use on the factory floor.
As an example, Renik et. al. (U.S. Pat. No. 4,649,556) simulated the conditions that would be encountered in a temper mill when measuring grain size on-line with an X-ray diffraction apparatus. The steels used in the simulation were AISI 1006 and 1008 drawing quality steels optically measured for grain size by the grain intercept method. To achieve an acceptable standard error of 20% in the X-ray determined grain size measurement for a sample moving at 1500 feet per minute and at a useful counting rate for existing technology, 100 non-overlapping segments would have to be examined. For steel moving at 4500 feet per minute which is not uncommon at the coiler, the number is 300. In this case the total analysis time for one grain size determination would be ten seconds, and in that time frame the steel strip would have moved over 1/2th of a mile. This means that the spatial resolution of such a system is 750 feet of steel strip product. None of these proposed systems is currently in use.